This invention pertains to porosity measuring and, in particular, to a porosity measuring apparatus for providing porosity control for perforating systems.
Perforating systems have been widely used for perforating a variety of sheet materials. One such sheet material is paper used in the construction of cigarettes. In perforating cigarette paper and, in particular, cigarette tipping paper, it is essential that the porosity of the resultant perforated paper be strictly controlled. In this manner, porosity variation and, correspondingly, variation in dilution of resultant cigarettes can be held within desired limits.
The porosity of a web of sheet material undergoing perforation can vary due to variations in web characteristics as well as variations in operating characteristics of the particular perforating system being used. A number of techniques have been devised for monitoring web porosity and for using the monitored porosity to control system operation to obtain a desired porosity. U.S. Pat. No. 4,025,752 describes one such technique wherein the porosity of a moving web perforated by electric sparks is monitored. A vacuum is applied to a chamber provided with an opening in facing relationship to the web. Variations in web porosity result in changes in the vacuum established within the chamber. These changes in vacuum cause corresponding changes in pressure which are monitored by a pressure detector.
The output electrical signal of the pressure detector is compared with a preselected or preset electrical signal representative of desired web porosity. The resultant difference signal is further processed by amplification and by multiplication with a signal representative of web speed. Circuitry for controlling the pulse generator at the electric spark perforating station is responsive to the processed signal and provides electric discharges which result in a web porosity which tracks desired web porosity in typical control system fashion.
U.S. Pat. No. 4,121,595 discloses another technique wherein web porosity is measured indirectly and utilized to control system components to ensure desired porosity. In this case, a moving web of tipping paper is perforated in a row pattern by a perforating assembly comprising a laser beam and cooperating optics. The perforated paper is then applied to the filter plug region of cigarette rods of double unit length which are subsequently severed to form individual cigarettes. The individual cigarettes are conveyed to a testing station whereat the average permeability or porosity of the cigarettes is measured and the measured porosity compared to a desired range of porosity values. Deviation from this desired range results in a signal which is used to vary the characteristics of the perforating laser beam so as to provide perforations which cause the porosity of the resultant cigarettes to lie within the desired porosity range.
In this system, the testing station comprises a testing chamber at which testing fluid from a constant pressure source is passed through the cigarette under test. A transducer having a diaphragm is responsive to the pressure of the fluid as it leaves the cigarette and this diaphragm controls a capacitor which supplies signals to an integrating circuit. The output of the integrating circuit corresponds to the average permeability or porosity of the cigarettes and is coupled to a comparator which receives a reference signal from an adjustable potentiometer. The comparator output signal is applied to a servomotor which adjusts a resistor in series with the source supplying energy for generating the laser beam.
The system of the last mentioned patent alters web porosity as a function of resultant cigarette porosity and measures the latter through application of fluid under pressure. U.S. Pat. No. 3,258,117 discloses an alternate procedure for measuring cigarette porosity or permeability in which a fluid maintained at a constant flow is pulled through the cigarette and the pressure drop across the cigarette is measured and compared with the pressure drop across the standard through which the same flow is applied. In this case, the pneumatic system includes a vacuum pump coupled to a surge tank. The surge tank feeds a conduit having first and second needle valves for providing fine and course adjustement of the flow rate. A holder coupled to the conduit receives the cigarette to be tested and the pressure drop across the cigarette at the constant flow rate is automatically measured by a manometer. A valve in the conduit permits coupling the flow to a standard whose pressure drop is also read. Deviations of the pressure drop across the cigarette from the pressure drop across the standard indicate deviations in porosity or permeability from the preselected porosity of the standard.
It is a broad object of the present invention to provide an improved apparatus for measuring the porosity of a moving web of sheet material.
It is a further object of the present invention to provide a perforating system for perforating a moving web of cigarette paper including improved apparatus for measuring and controlling the porosity of the moving web.